The present invention relates to the field of RF assembly components. More specifically, the present invention relates to ground-plane interfaces between RF assembly components.
In the design and implementation of microwave and millimeter-wave radio-frequency (RF) assemblies, there is often a need to interface component sub-assemblies, e.g., a gallium arsenide (GaAs) power amplifier with attendant heat sink to a printed-circuit board assembly. At these wavelengths, it is desirable that the interfaces between two components have electronically contiguous grounds. That is, the ground planes associated with any given signal should have substantially zero impedance over the inter-component interface.
Utilizing conventional interfacing techniques, the ground of one component is coupled to the ground of another component through the use of seam soldering, specialized contact assemblies, or direct grounding-member contact. All such conventional interfacing techniques present problems at short wavelengths.
With seam soldering, a grounding member of a first component is positioned adjacent a grounding member of a second component so as to produce a seam between the grounding members in the form of a small crack. This seam is then soldered over a significant portion of its length. That is, solder is flowed into and over the crack, thereby producing essentially a contiguous grounding member providing an acceptable electrical ground interface between the components.
Unfortunately, seam soldering is a labor-intensive production procedure. Also, a significant risk of component damage from thermal stress exists with this procedure. The use of seam soldering therefore adds significantly to production costs while simultaneously reducing production yield.
In addition, the use of seam soldering renders an assembly difficult or impossible to repair. In the event of component failure, the entire assembly, rather than the defective component, often requires replacement. This significantly increases the costs of repair and maintenance. Where the assembly is not easily accessible, as in a satellite, the cost increase may become prohibitive.
Where the ability to effect component replacement justifies the additional costs involved, specialized contact assemblies may be used. With specialized contact assemblies, a contact assembly is affixed to each component and mated to provide the requisite interface. Through the use of specialized contact assemblies, relatively good impedance matches are provided, minimizing discontinuities.
Unfortunately, specialized contact assemblies require additional components and production steps, thus increasing production costs. In addition, the interfaces between the components and the specialized contact assemblies must be accounted for in the design of the components. This may increase the complexity of component design, again increasing overall assembly costs.
Also, just as a plug-and-socket assembly is unlikely to be as electrically sound as a continuous wire, specialized contact assemblies are unlikely to be as good as a continuous ground plane and circuit. This is increasingly true as wavelength decreases. To compensate for interconnection problems, such specialized contact assemblies may be formed of or incorporate precious metals and other costly materials. Similarly, the specialized contact assemblies may also or alternatively incorporate costly interfacing techniques. Such materials and/or techniques further increase the cost of assembly production. Moreover, such materials and/or techniques often require significant soldering operations that risk thermal damage to other subassembly components.
The simplest and least expensive method of providing a ground interface between components of an RF assembly is to provide direct grounding-member contact. In this case, the grounding members of the components are machined, cast, or otherwise formed to be mechanically complimentary, i.e., to fit together snuggly. The grounding members are then mechanically coupled, typically by means of one or more screws, so as to provide the necessary mechanical and electrical contact.
Since direct grounding-member contact requires no solder or additional components, the ability to effect repairs in the field by the replacement of components is maximized. Often, such replacements may be effected by personnel with minimum training and utilizing no special tools. Direct grounding-member contact is therefore extremely cost effective.
Unfortunately, direct grounding-member contact has its own problems. For example, thermal and vibrational stresses may cause the contact to loosen. Corrosion, from either contamination or electrolytic action, may affect contact quality. A loose or otherwise poor contact may cause severe discontinuity at the interface, resulting in unpredictable impedance characteristics.
Additionally, a loose or poorly fitting interface between grounding members may result in gaps or holes. At microwave and millimeter-wave frequencies, such gaps or holes may act as waveguide stubs of indeterminate lengths. Such stubs produce echoes, ringing, and other noises that may severely degrade the signal.
What is needed, therefore, is a method of interfacing components of microwave or millimeter-wave assemblies that allows efficient, cost-effective production of the assemblies. This assembly method should be labor-conservative, should not subject the components to undue thermal stresses, and should not involve the use of additional devices beyond the components themselves. This assembly method should result in an assembly that is easily reparable in the field in the event of component failure without requiring either specialized tools or training.